1. Field of the Invention
The present invention relates to an optical encoder with a resinous code plate.
2. Description of the Related Art
An optical encoder generally includes a light emitting section, a light receiving section, and a code plate disposed movably relative to the light emitting section and the light receiving section. The code plate is provided with a code pattern for converting a light input emitted from the light emitting section into a coded-light output to be output toward the light receiving section. This type of optical encoder serves to sense, at the light receiving section, the coded light output as a light transmitted or reflected through the code pattern, so as to detect the displacement of the code plate, and is used for detecting the position or speed of a moving object.
For example, Japanese Unexamined Patent Publication (Kokai) No. 11-287671 (JP11-287671A) discloses an optical encoder with a code plate, the code plate including, on one surface thereof, a plurality of tracks respectively provided with code patterns different from each other. The tracks of the code plate are provided with code-pattern surfaces different from each other, for converting respectively a light input into various coded-light outputs, and are formed on the surface of the code plate in a side-by-side arrangement (e.g., a concentric arrangement) exhibiting a mutually identical extending direction. The code-pattern surface of each track includes a flat surface area and an irregular surface area, adjacent to each other as seen in the extending direction of the track.
The code plate is generally constructed in such a manner that a metal coating, such as a chrome coating, is deposited onto the surface of a glass plate so as to provide a bright/dark code pattern. On the other hand, a code plate formed from a resinous or plastic material (referred to as a resinous code plate, in the present application) is known, which possesses an advantage of mass production at a low cost due to the use of an injection molding technique.
In order to form the above-described code-pattern surface on the resinous code plate, it is conventionally performed to process or form patterned areas, corresponding to the flat surface area and the irregular surface area, on the molding surface of a mold used for molding the resinous code plate from a molten resinous material, and to transfer the shape of the molding surface to the resinous material. However, in the case where an insufficient filling pressure is applied to a resinous material having a low flowability in a molten state, the shape of the molding surface of the mold may be inaccurately transferred to the resinous material.
For example, in the case where the irregular surface area includes a V-groove array, the molten resinous material flows in the mold in a direction corresponding to the longitudinal direction of the respective V-grooves. In this connection, if the irregular surface area of one track is arranged side-by-side relative to the flat surface area of the other track as seen in the flowing direction of the molten resinous material, the filling pressure of the flowing molten resinous material may locally fluctuate about a portion corresponding to the boundary between the irregular surface area and the flat surface area, which may result in incomplete V-grooves.
If the code pattern of the track of the resinous code plate is incompletely formed, the detection accuracy of the optical encoder may be affected.